Refrigeration system

ABSTRACT

A refrigeration system having at least one defrostable cold-production device is disclosed, beneath which device a drip tray provided with a drain channel is arranged. In such a refrigeration system, it is desirable to reduce the risk of flooding because of a blocked drain channel. For that purpose, a liquid sensor which detects the presence of liquid at a level above the drain channel is provided in the drip tray.

The invention relates to a refrigeration system having at least onedefrostable cold-production device, beneath which device a drip trayprovided with a drain channel is arranged.

Such a refrigeration system is known from EP 0 291 381. The drip tray isespecially important in refrigeration systems which are repeatedlydefrosted either periodically or in dependence on their load. The waterdraining off during defrosting is caught in the drip tray and dischargedthrough the drain channel. The main application of such refrigerationsystems is in refrigeration appliances, for example, refrigerators,upright freezers, chilled display cabinets or counters, and chestfreezers. Such refrigeration appliances are in particular affected bythe problem that the drain channel becomes blocked, for example, by foodparticles, bits of packaging, dust or other foreign bodies. In thatcase, the water is unable to drain out of the drip tray or is not ableto drain out quickly enough. Sooner or later the drip tray will thenoverflow. Since in most cases the amount of water from defrosting isgreater than the capacity of the drip tray, this leads to flooding inthe refrigeration appliance, to the detriment of the products storedtherein.

The drain channel therefore has to be periodically cleaned. For thatpurpose, in refrigerators which are closed by a door (EP 0 156 229 A2),it is known to construct the drain channel in the manner of a nozzle,that is, to give it a conical construction which converges towards awall in such a manner that a passage of reduced cross-sectional area iscreated. As the door of the refrigerator is opened, a current of air issucked through the nozzle-like opening and clears the opening of dirtand dust. This solution is not applicable, however, to largerrefrigeration systems, chest freezers or similar appliances. As a rule,a door is not opened and closed here.

The invention is based on the problem of reducing the risk of floodingduring defrosting.

This problem is solved in a refrigeration system of the kind mentionedin the introduction in that a liquid sensor which detects the presenceof liquid at a level above the drain channel is provided in the driptray.

When the drain channel is blocked, liquid that is produced, for example,during defrosting, will collect in the drip tray. As soon as it hasreached a level or a depth at which it can be detected by the liquidsensor, the liquid sensor can give a warning signal or interrupt thedefrosting. By means of the warning signal, an operator can be madeaware that the drain channel is blocked. Normally, only a few manualoperations are required to clean the drain channel. The warning signalcan comprise, for example, illumination of a lamp.

Although a liquid sensor in a refrigeration system is known from DE 3326 799 A1, this sensor is arranged in the drain channel itself. It isintended to interrupt the operation of the compressor as long as wateris flowing out of the drip tray, that is, during the defrosting process.In this manner, it is intended that the layer of ice or frost on therefrigerating surface is fully defrosted, but the pauses during coldproduction are not too long. This sensor is completely unsuitable forthe problem under consideration, however, because any blockage of thedrain channel that may occur would be interpreted by the sensor to meanthat defrosting was over and the melted water had drained off, whereasin reality the drip tray is full of water. The risk of flooding cannotbe influenced in any way by this.

The liquid sensor is preferably in the form of a heatable temperaturesensor. The influence of heating on the temperature is different in airto that in water or ice. Using the temperature, one can thereforedetermine during or after heating whether there is still water or ice inthe drip tray or whether there is only air in the drip tray.

The liquid sensor preferably comprises an electrical resistor. Heatingby means of current through an electrical resistor can be regulatedrelatively well and can be controlled without difficulty underconditions of safety.

It is also preferred for the liquid sensor to be surrounded by aprotective guard. It has been shown that the actual ambient temperaturehas only a very minor influence on the thermal behaviour of the sensorduring heating. More critical, however, is the supply or dissipation ofheat by passing air. The protective guard prevents the sensor beingaffected by circulating air. The result is consequently more reliable.

It is especially preferred herein for the protective guard to be in theform of a capsule, the wall of which has openings for the passage or iceand/or water. In this way, on the one hand any appreciable circulationof air avoided, but at the same time the passage of ice and/or water isallowed. The liquid sensor is therefore in fact restricted to monitoringwhether air or water or ice is present in the drip tray.

In a preferred embodiment, a control device is provided, whichdetermines the temperature at the liquid sensor at at least twodifferent times and compares the temperatures with one another. In thismanner several items of information can be obtained. Firstly, theactual, that is, absolute, temperature rise or temperature drop can beestablished. Secondly, however, the relative temperature increase or therelative temperature drop can also be ascertained, by relating thetemperature difference between the two values to one of the twotemperature values. The relative temperature change provides a clearindication as to whether the liquid sensor is surrounded by water or iceor by air. In water and ice, the relative temperature change is greaterthan in air.

Preferably, the control device has a memory for a first temperaturevalue, a divider for forming a quotient from a second temperature valueand the first temperature value, and a comparator, which produces analarm signal when the quotient exceeds or falls below a predeterminedvalue. Whether that value is exceeded or fallen below is determined bywhether the larger or the smaller temperature value is used as thecounter. If, for example, the smaller temperature value, which isdetermined after a certain cooling-down phase, is used as counter, oneassumes that the liquid sensor is located in air if the quotient isgreater than 0.3. It can be assumed that the sensor is surrounded byliquid if the quotient is 0.04. In order to determine reliable thresholdvalue, 0.1 or 10% can be chosen for example, below which an alarm is setoff.

The control device preferably has a timer which initiates a measuringcycle at predetermined intervals. Measurement is therefore effected onlyfrom time to time, for example, every 15 minutes.

In that case, the control device preferably also controls the heating ofthe liquid sensor. The coordination between the heating and thetemperature measurement is consequently simplified. In particular,temperature measurements can thus taken in dependence on the duration orthe starting and finishing points of the heating.

The resistor preferably has a temperature-dependent resistance value.The control device can thus measure the temperature virtually at thesame time as it measures the heating. It is possible here to use eithera resistor with a positive temperature dependency or a resistor with anegative temperature dependency (PTC- or NTC-resistor). An electricalheating output of about 0.5 W has proved suitable when using a Pt-100resistor.

It is preferred for one measurement to be taken in the unheated stateand another to be taken at the end of a heating period of predeterminedlength. Measurement of the temperature in the unheated state thereforeprovides information about the ambient temperature at the same time,provided that the last heating period has run for a sufficiently longtime. The relative temperature change can therefore be related to theambient temperature, which allows for improved interpretation of data.Measuring the temperature at end of the heating period on the one handallows information to be obtained about the final temperature resultingfrom the heating. The final temperature in air is substantially higherthan in water or ice, all other conditions being identical. On the otherhand, it allows information to be obtained about the rise intemperature.

The heating period preferably lasts for about 3 minutes. Although thistime is not sufficient to reach the actual final value which wouldobtain with a corresponding heating over a longer period, a heatingduration of 3 minutes still results in a sufficient increase intemperature to give temperature values that differ sufficiently from oneanother for them to be evaluated.

Measurement preferably takes place in the unheated state prior tomeasurement at the end of the heating period. In that case, measurementin the unheated state also provides information about the ambienttemperature. Measurement at the end of the heating period then providesinformation about the temperature rise during the heating. This issubstantially quicker in water or ice, but reaches only a smaller finalvalue than in air.

In another preferred embodiment, measurement in the unheated state takesplace a predetermined time after the end of the heating period. Here,cooling of the temperature sensor is ascertained. Cooling is alsosubject to substantially the same laws as heating. Both processes followsubstantially an exponential characteristic. Conclusions about themedium that surrounds the liquid sensor can therefore also be drawn fromthe progression of cooling.

The predetermined time is preferably about 1 minute. Cooling issubstantially quicker than heating, so that a shorter time is sufficientfor measurements to be taken with the necessary reliability.

In an especially preferred embodiment, three measurements are taken, themiddle measurement being taken at the end of the heating period and theother two being taken before and after without heating. With this kindof measurements, additional information is obtained, for example, aboutthe initial temperature, which corresponds in the majority of cases tothe ambient temperature. The more information is available, the greateris the meaningfulness and the reliability with which the results areachieved.

Measurements are advantageously taken outside the defrosting period.Outside the defrosting period there should be no liquid in the driptray. Should liquid nevertheless be present, this indicates that thedrain channel is blocked. The warning signal can now be actuated or thenext defrosting process can even be delayed until the fault isrectified.

The liquid sensor is advantageously fixedly adhesively secured in thedrip tray by a layer of adhesive. The layer of adhesive can be formed,for example, by an adhesive strip. This facilitates fixing. Noadditional bores or the like require to be provided in the drip tray.

The invention is described hereinafter with reference to a preferredembodiment in conjunction with the drawing, in which

FIG. 1 is a diagrammatic view of a refrigeration system,

FIG. 2 is a view of a liquid sensor, partly in section,

FIG. 3 is a temperature curve in air and

FIG. 4 is a temperature curve in water/ice.

A refrigeration system 1, here illustrated as a refrigerator, has; acabinet 2 which encloses a refrigeration space 3. Access to therefrigeration space 3 is through a door 4.

A cold-production device, for example, an evaporator 5, which in knownmanner is supplied from a compressor with refrigerant in liquid form, isarranged in the refrigeration space 3. The compressor is not shown.

As is known, during operation a layer of ice or frost forms on theevaporator 5 in the course of time, which hampers the transfer of heatbetween the evaporator 5 and the refrigeration space 3. The evaporator 5therefore has to be defrosted periodically.

The water draining off during defrosting of the evaporator 5 is caughtin a drip tray 6 and discharged from the refrigeration space 3 through adrain channel 7. It can then, for example, flow into the waste waterdisposal system or be conveyed to a heated surface of the compressor,where it is able to evaporate again.

Occasionally, the drain channel 7 may become blocked. In that case, thewater would not be able to drain out of the drip tray 6. It wouldcollect there and, when a certain amount was exceeded, would overflowand lead to flooding on the floor of the refrigeration space 3.

In order to be able to recognize this danger situation in good time, aliquid sensor 8 fixed in the drip tray 6 by an adhesive strip isprovided, the sensor being able to detect liquid in the drip tray 6 at alevel above the drain channel 7. The liquid sensor 8 is connected to acontrol device 9 which actuates a warning device 10 when it establishes,by means of the liquid sensor 8, that liquid, for example, water, hascollected in the drip tray 6. The warning device can be, for example, awarning light or an acoustic signalling means, for example, a hooter.The control device 9 can, of course, also be constructed so that itcontrols a defrosting device, not shown more specifically, for theevaporator 5. Whenever liquid is detected in the drip tray 6, defrostingof the evaporator 5 is at first not initiated at all, rather, simply awarning is given.

The construction of the liquid sensor 8 is illustrated in greater detailin FIG. 2. The main element of the liquid sensor is a Pt-100 resistor11, which is connected by way of an electrical lead 12 to the controldevice 9. The resistor 11 is surrounded or encapsulated by a housing 13.However, the housing 13 has openings 14 at its two sides and an opening15 at its end face. The housing 13 prevents moving air from causingtransfer of heat to or from the resistor. The openings 14, 15nevertheless allow entry of water to the inside of the housing 13. Thearrangement of the openings 14, 15 is therefore dependent on the desiredposition and orientation of the liquid sensor 8 in the drip tray 6.

The resistor 11 has a temperature dependency, that is, its resistanceincreases as the temperature increases. The control device 9 istherefore able to detect the temperature at the resistor 11 simply bymeasuring the resistance value.

To determine whether liquid is present in the drip tray 6 or not, thecontrol device 9 supplies the resistor 11 with an electrical heatingpower of about 0.5 W. Referring to FIGS. 3 and 4, the difference in thetemperature gradient between air (FIG. 3) and water or ice (FIG. 4) isclearly apparent. At a time t₁ the control device 9 starts to heat theresistor 11. Heating lasts until the time t₂. In this particular casethis is about 3 minutes. Starting from identical ambient temperaturesTL1 for air and TW1 for water, the temperature rises to a value TL2 whenthe liquid sensor 8 is in air, and to a value TW2 when the liquid sensor8 is in water or ice. The temperature TL2 in air is here substantiallyhigher than the temperature TW2 in water. The temperature rise initiallyis much steeper in water than in air. Either the absolute temperatureTL2 or TW2, or the temperature difference TL2-TL2 or TW2-TW1, or therelative temperature increase can now be used as the criterion todetermine whether the liquid sensor 8 is surrounded by water or by air.If desired, one need not wait for the entire heating period to the timet₂. Temperature measurement can alternatively be effected earlier.

At the time t₂, heating is stopped. By virtue of the lower temperaturein the refrigeration space 3, the resistor 11 cools, namely, with thecharacteristics illustrated in FIGS. 3 and 4 which correspondsubstantially to an exponential characteristic. At the time t₃, which isabout 1 minute after the time t₂, a temperature TL3 in air and TW3 inwater is reached. The temperature TL3 in air is higher than thetemperature TW3 in water. Either the absolute temperature TL3 or TW3 canbe used as criterion for whether there is water or air in the drip tray,or the absolute temperature difference can be used, which is greater inair than in water, or the relative temperature difference can be used,that is, the quotient from the absolute temperature difference and thelarger or smaller of the two values at the times t₂ and t₃ respectively.If the relative temperatures are used, one can establish, for example,how high the temperature at the time t₃ is in relation to thetemperature value at the time t₂. In water, the temperature will only beless than 4% of the value TW2. In air, the temperature TL3 is normallygreater than 10% of the temperature TL2. This 10% can therefore be takenas the limit value. If the temperature at the time t₃ is less than 10%of the value at the time t₂, an alarm is set off.

Finally, all three temperatures TL1, TL2 and TL3, and TW1, TW2 and TW3can be evaluated. The temperatures TL1 and TW1 measured at time t₁ hereprovide information about the ambient temperature, whilst the othertemperatures t₂ and t₃ provide information about the temperaturedevelopment, which in its turn again allows conclusions to be drawnabout the environment around the liquid sensor 8.

Measurement is preferably effected outside the defrosting periods.Whenever liquid is then detected in the drip tray 6, further defrostingprocesses can for the time being cease until the blockage in the drainchannel 7 has been eliminated.

The control circuit also contains a timer, for example a clock, so thatmeasurement can be effected repeatedly every 15 minutes. Other intervalscan be selected.

On the one hand this saves energy, but on the other hand also preventsunnecessary heating of the refrigeration space. It is impossible toexclude, of course, that liquid will collect in the drip tray within theinterval. But it is highly likely that the fault will be noticed in goodtime during a measurement outside the intervals.

We claim:
 1. A refrigerating system having at least one defrostablecold-production device, beneath which device a drip tray provided with adrain channel is arranged, in which tray a temperature dependentresistor is located as a liquid sensor, electric power being provided tosaid resistor to heat it and means being provided for determining achange in resistance value of the temperature dependent resistor independence on defrost water level in the drip tray, and including acontrol device having means to determine the temperature at the liquidsensor at at least two different times and to compare the temperatureswith one another.
 2. A refrigeration system according to claim 1, inwhich the liquid sensor is surrounded by a protective guard.
 3. Arefrigeration system according to claim 2, in which the protective guardis in the form of a capsule having a wall which has openings for thepassage of ice and/or water.
 4. A refrigeration system according toclaim 1, in which the control device has a memory for a firsttemperature value, a divider for forming a quotient from a secondtemperature value and the first temperature value, and a comparator,which produces an alarm signal when the quotient exceeds or falls belowa predetermined value.
 5. A refrigeration system according to claim 1,in which the control device includes a timer which initiates a measuringcycle at predetermined intervals.
 6. A refrigeration system according toclaim 1, in which the control device also includes means to control theheating of the liquid sensor.
 7. A refrigeration system according toclaim 1, in which the means to determine takes one measurement in anunheated state and another measurement at the end of a heating period ofpredetermined length.
 8. A refrigeration system according to claim 7, inwhich the heating period lasts for about 3 minutes.
 9. A refrigerationsystem according to claim 7, in which measurement takes place in theunheated state prior to measurement at the end of the heating period.10. A refrigeration system according to claim 7, in which measurement inthe unheated state takes place a predetermined time after the end of theheating period.
 11. A refrigeration system according to claim 10, inwhich the predetermined time is about 1 minute.
 12. A refrigerationsystem according to claim 7, in which three measurements are taken, amiddle measurement being taken at the end of the heating period and theother two measurements being taken before and after without heating. 13.A refrigeration system according to claim 1, in which measurements aretaken outside a defrosting period for the cold-production device.
 14. Arefrigeration system according to claim 1, in which the liquid sensor isfixedly adhesively secured in the drip tray by a layer of adhesive.